the new man in charge with a personal energy that hasn't been seen in that role in decades. >>> fighting alzheimer's. a stunning new look at just how many americans have been stricken and the new way doctors are combatting the effects with a kind of pacemaker for your brain. >>> and the new warning from nasa about something we right now can't do anything about. the asteroids they call city killers in the asteroid business. "nightly news" begins now. >>> good evening. an explosion lit up the desert in nevada. the kind of thing we see in a war zone. it is why they train there for deployment in war zones, but this was stateside training. something went wrong at the hawthorne army depot not far from the california border. in the end, seven american families got the worst possible news. seven u.s. marines killed in a mortar accident. others were wounded. the investigation obviously under way now into what went wrong. nbc's miguel almaguer is with us from the scene tonight. miguel, good evening. >> reporter: brian, good evening. the hawthorne army depot has been used for decades to store ammun